Electronically tunable matching circuit for circulators

ABSTRACT

A novel matching circuit for a circulator of the type requiring a resistance and capacitance for tuning and matching the circulator. The matching circuit includes an electronically variable resistance and capacitance means for enabling optimum performance of the circulator over the range of its operating frequencies.

i United States Patent Simmons [45] July 1 l, 1972 s41 ELECTRONICALLY TUNABLE 3,310,759 3/1967 Ogasawara ..333/1.1 MATCHING CIRCUIT FOR 3,475,700 10/1969 12ml .333/7 CIRCULATORS OTHER PUBLICATIONS [721 wm'am 51mm, Califlsoductor Circuit Design, Melals Appl. Note 8- 195, Man, [73] Assignee: Communications Satellite Corporation 1968' sheet & P 1 relied Filed: y 1970 Primary Examiner-Paul L. Gensler [21] APP] N04 33,762 Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT 6 A novel matching circuit for a circulator of the typ requiring [58] Field 333/1 1 24 2 328,155 a resistance and capacitance for tuning and matching the circulator. The matching circuit includes an electronically variable resistance and capacitance means for enabling optimum [56] References Cited performance of the circulator over the range of its operating UNITED STATES PATENTS frequencies- Rudd, Jr. et al ..328/l55 X lClaim,2DrawlngFigures Patented July 11, 1972 3,676,803

MATCHING MATCHING CIRCUIT CIRCUIT I 2 MATCHING I CIRCUIT PRIOR ART INVENTOR WILLIAM J. SIMMONS ATTORNEYS ELECTRONICALLY TUNABLE MATCHING CIRCUIT FOR CIRCULATORS BACKGROUND OF THE INVENTION The invention relates to circulators and specifically to circulators adapted to be used as isolators. A circulator is an N port device, operable at microwave frequencies, where N is greater than 2. It has the property that a signal applied to a first port emerges at a second port with a minimum of input power attenuation while negligible power emerges from the other ports. One type of prior art circulator is the junction or lumped element type. Examples of such circulators are disclosed in the patents to Konishi, entitled Lumped Element Y Circulator, U. S. Pat. No. 3,335,374, and Roberts, entitled Ferrite Circulator Having Three Mutually Coupled Coils Coupled to the Ferrite Material, U. S. Pat. No. 3,286,201. Circulators of this type have a ferrite core located at the junction of a plurality of conductors. The ferrite core, which acts as a resonant cavity, is magnetized by means of a magnet. Magnetization of the ferrite causes a change in its permeability. It is this variation in the permeability of the ferrite which causes power entering one port to emerge at another with a minimum of attenuation while negligible power emerges from the other ports. Depending upon the direction of the magnetic bias applied to the ferrite core, the power will pass through the core in a clockwise or counter clockwise direction.

When a matching resistor is connected to one or more ports of an N port circulator, an isolator may be realized. Taking as an example a three port circulator having a clockwise orientation, the connection of a matching resistor to one port creates an isolator having one input and one output port. In the operation of such a device a signal applied to the input port travels in a clockwise direction through the core and emerges from the output port with a minimum of attenuation while a signal reflected from the output port is absorbed by the matching resistor connected to the third or next port in a clockwise direction and does not appear at the input or subsequent ports in a clockwise direction.

Depending upon the operating frequencies, the characteristic impedance of the isolator and lengths of transmission line connected to each port, tuning of an isolator is accomplished by connecting either a capacitive or inductive matching circuit at each of the ports. The matching circuits may take the form of capacitive networks if the isolator exhibits predominately inductive reactance, tuning being accomplished by varying the effective capacitance of each network. An isolator exhibiting capacitive reactance is tuned by varying the effective inductance of the matching circuit.

FIG. 1 of the drawings shows a prior three port isolator. Three matching circuits 1, 2, and 3 are used to tune the isolator which is assumed to exhibit an inductive impedance at each port. Matching circuits 1 and 2 may comprise purely capacitive networks to match the input and output ports to the respective input and output transmission lines. Matching circuit 3 includes a matching resistor R to absorb the power reflected from port 2. With reference to matching circuit 3, optimum operation of the isolator requires that capacitor C, resonate with the isolators characteristic inductance and that resistor R be equal to the real part of the characteristic impedance (or, gyration resistance) of the isolator. Both of these parameters, the gyration resistance and the characteristic inductance, depend upon the frequency and permeability of the core. Mathematically, C must be chosen to satisfy the equation:

where m operating frequency I characteristic inductance of the circulator looking into the isolated port.

With reference to matching circuits 1 and 2, these must match the impedance of the input and output terminals of the circulator to their associated transmission lines. This may be done by varying the effective capacitance of the matching circuits, providing that the range of operating frequencies as well as the length of transmission line between the circulator and matching network are chosen to make the isolator appear inductive.

Prior isolators have been found deficient in two areas. Tuning has been accomplished by using mechanically adjustable air variable capacitors in networks coupled between the conductors associated with the ferrite core and ground. Tuning of the prior art devices is accomplished by physically moving the plates of the tuning capacitors. The second problem area relates to the matching resistor. Fixed resistors used in the prior art resulted in an isolator having only one optimum frequency of operation dependent on the value of the fixed resistor. In addition, since the exact value of the matching resistor for optimum circuit operation can only be determined experimentally, prior art devices require a time consuming trial and error method in choosing the proper resistor.

SUMMARY OF THE INVENTION The above limitations on the use of prior isolators have been removed by the applicants invention. The invention relates to means for terminating an isolator so that it may perform over a broad range of frequencies while achieving optimum tuning and matching. The invention uses electronically variable capacitance and resistance means operable at microwave frequencies to effect optimum tuning and matching over wide range of frequencies.

Additionally, the value of the matching resistance means for optimum circuit operation can be easily determined experimentally by varying this resistance.

Another feature of this invention is that the optimum matching can be accomplished electronically by using, as the variable resistance, a diode whose resistance varies as a function of its forward bias.

Still another feature is the use of an electronically tunable variable capacitor in the form of a varactor diode for optimum electronic tuning. As is known in the art, the value of the capacitance of the varactor varies as a function of its reverse bias. Therefore, the isolator can be tuned over a range of frequencies simply by varying the bias to the varactor.

Another feature is to use as the variable resistance diode, a PIN diode. Since the resistance of such a diode varies as a function of its forward bias, optimum matching over a range of frequencies can be accomplished simply by varying the bias to the PIN diode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a circulator and prior art matching circuit means as described above.

FIG. 2 is a partial schematic diagram of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The invention pertains to a novel method of terminating a circulator adapted to be used as an isolator and having an inductive characteristic impedance. Circulators which can be used with the invention are known in the art and have been previously described. Specifically, the preferred embodiment of the circulator which is utilized is a three port device consisting of a ferrite core located at the junction of three microwave conductors. Electronically tunable matching circuits are coupled to each of the ports. These matching circuits include a first diode whose capacitance varies as a function of its bias and at least one matching circuit includes a second diode whose resistance varies as a function of its bias. Using electronically tunable elements in the matching circuit allows for convenient adjustment of the circuit over a wide range of frequencies without mechanically disturbing the device. Additionally, the matching circuit may be made to track any frequencies within the operating range of the isolator, giving optimum performance at each frequency. This may be done by allowing the value of the diodes bias to follow the frequency of the isolator. Since a means for accomplishing this tracking is not part of this invention, a detailed description of this means will not be included herein.

With reference to FIG. 2, matching circuits A, B, and C are each coupled respectively to ports a, b, and c. Since each of these circuits is identical, only matching circuit C will be described in detail.

Matching circuit C includes a variable resistance means which may be a PIN diode 1c, a variable capacitance means, which may be a varactor 2c, and means for connecting the variable resistance and capacitance to sources of variable bias. A variable bias source 10 is connected via a choke coil 12 to the anode of PIN diode 1c; the cathode being connected to ground. The cathode of the varactor 2c is connected to a variable bias source 14 through a choke coil 16. The anode of varactor 2c is also connected to ground. Diode 1c and varactor 2c are connected to port through coupling capacitors l8 and 20, respectively, which prevents the DC. voltages from bias sources and 14 from interfering with the operation of the device.

The resistance R of PIN diode 1c varies as a function of its forward bias according to the following relationship:

where l= forward bias current and K and M are constants. PlN diodes such as are used in the preferred embodiment of the invention are readily available on the market.

To realize maximum absorption of the power reflected into port 0, the resistance of diode 1c is adjusted by varying bias means 10 until the effective resistance of the circuit is equal to the gyration resistance of the isolator. Since the gyration resistance of an isolator is frequency dependent, the value of resistance selected to achieve maximum isolation is also frequency dependent. With the present invention the resistance can simply be varied by varying the bias applied to the PIN diode.

The varactor 2c is a diode whose capacitance varies as a function of the reverse bias applied thereto. The effective capacitance of the matching circuit C is effectively adjusted electronically simply by adjusting the bias 14. By varying the value of varactor 2c the effective capacitance of the circuit is easily made to resonate with the characteristic inductance of the isolator for any frequency over the range of operation of the isolator.

Matching circuits A and B are adjusted to obtain maximum power flow between port a and port b and therefore a variable resistance is optimally set at a low or even zero value to minimize power losses. On the other hand, matching circuit C is adjusted so that port c is matched for maximum power absorption which normally requires a high resistance. Since the forward power loss consists of a dissipative loss due to the isolator plus a frequency sensitive mismatch loss, proper tuning of matching circuits A and B will minimize this loss. Similarly, since the amount of reflected power absorbed by matching circuit C is frequency dependent, proper adjustment of the impedance of this circuit will effect maximum power absorption The values of the components used will vary depending upon the particular circulator, the operating frequencies and lengths of transmission line used. However, in each case, they can be easily determined by anyone of ordinary skill in the art by applying the teachings of this invention.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and changes in form and details may be made therein without departing from the spirit and scope of the invention.

[claim 1. An electronically tunable isolator comprising a ferrite circulator having more than two ports, and a first matching circurt connecte to at least one port, said matching circuit comprising a varactor diode, a pin diode, first and second choke coils, first variable biasing means connected to said varactor diode through said first choke coil, second variable biasing means connected to said pin diode through said second choke coil, and means including coupling capacitors for connecting said varactor diode and said PIN diode to said one port, said isolator further comprising additional matching circuits each including a varactor diode and a PIN diode connected respectively to the remaining ports of said isolator, said additional matching circuits being tuned to give maximum power flow through said ports. 

1. An electronically tunable isolator comprising a ferrite circulator having more than two ports, and a first matching circuit connected to at least one port, said matching circuit comprising a varactor diode, a pin diode, first and second choke coils, first variable biasing means connected to said varactor diode through said first choke coil, second variable biasing means connected to said pin diode through said second choke coil, and means including coupling capacitors for connecting said varactor diode and said PIN diode to said one port, said isolator further comprising additional matching circuits each including a varactor diode and a PIN diode connected respectively to the remaining ports of said isolator, said additional matching circuits being tuned to give maximum power flow through said ports. 